1. Field of the Invention
The present invention relates to aluminum substrates suitable for use in the production of lithographic printing plates. More particularly, the invention pertains to a surfactant surface treatment for such aluminum substrates which improves the quality of radiation sensitive coatings subsequently applied to the substrate.
2. Description of the Prior Art
It has long been known to form printing plates by coating a lithographically suitable radiation sensitive composition onto the surface of an aluminum sheet substrate with subsequent imagewise exposure to radiation and development. Oleophilic image areas which remain after development accept and transfer ink during the printing process and the hydrophilic non-image areas accept water or aqueous solutions during printing to repel such greasy inks.
Typically the substrate comprises an aluminum alloy and the surface of the substrate has been pretreated before coating the radiation sensitive composition with one or more processes such as cleaning, degreasing, chemical etching, mechanical graining, electrochemical graining, anodizing and/or hydrophilizing. Each of these pretreatment steps contributes to forming a high quality printing plate by providing advantageous surface characteristics such as surface uniformity, adhesion to image areas, removal of non-image areas, length of printing run, surface hardness and differentiation between ink accepting and ink repelling areas.
In order to prepare a high quality printing plate, it is important that a very uniform radiation sensitive coating be applied to an underlying hydrophilic surface. Achieving such a uniform coating has been somewhat of a problem in the art and the resulting plates may show a somewhat irregular mottled or non-heterogeneous appearance. It has been found that this is a result of poor wetting of the radiation sensitive coating to the underlying hydrophilic surface. It has now been unexpectedly found that when a surfactant is included in the composition of an underlying layer, the applied radiation sensitive coating is substantially uniform and non-mottled in appearance.